Are these two paragraphs worded and structured correctly?

This is this the first two paragraphs of one of my novel summaries and I just wanted to make sure that they are all correct. Particularly the use of past tense with "had" and "since" in the final sentence.

In the time period after the Spellplague in 1385 DR, the lands of Calimshan were ripped asunder by the event known as the Second Era of Skyfire. Following the Spellplague, the Calimemnon Crystal, the magical prison of the efreet Memnon and the djinn Calim, shattered. The released creatures immediately resumed their hostilities that dated back to the First Era of Skyfire (circa –6500 DR). In the wake of the return of Calim and Memnon, many of the residents of Calimshan, presumed to be human, cast off their long held disguises and revealed themselves to be genasi. These descendants of the elemental powers pledged themselves to either fire or air, took over the cities of Memnon and Calimport respectively, and joined the war between the efreets and djinns.

The Second Era of Skyfire lasted until 1450 DR, when both Memnon and Calim were banished back to the Elemental Chaos by an unknown event. They left in their wake a devastated Calimshan, torn by the remaining djinns, efreets, and genasi warlords. The genasi of Calimshan were notorious for their treatment of humans as slaves and their constant fighting across the Calim Desert. The only bastion of order in the region was Almraiven, the last human-controlled city, which was ruled by the powerful archmage Acham el Jhotos, known as the WeavePasha. Once an intellectual center and home of magical research, Almraiven survived the Spellplague, and since that calamitous event had occurred the WeavePasha had worked tirelessly to protect the city from the ravages of the conflict between the djinns and efreets.